What is a Spam Filter?

Spam filters are algorithms that detect unsolicited, undesired, or infected emails and block those messages from reaching inboxes. There are a variety of spam filters, with detection capabilities ranging from basic pattern matching through to machine learning.

Spam filters do a lot of work. And much of it is complex, confusing, and constantly changing. With the pressure to keep viruses out and determine what’s unsolicited vs. solicited, it’s no wonder they sometimes don’t get everything right.

Don’t worry, though; you can do lots to ensure you’re playing by the rules and feeding spam filters the information they need to make an informed decision about your email—more on the steps you can take later.

There are three main types of spam filters: Email service provider, third-party, and desktop.

1. Email service provider spam filters

These are the spam filters you’re likely using without even knowing. Email service providers are the cloud-based services that most people use to send, receive and organize their emails. Think Gmail, Yahoo Mail, Office 365, and Hotmail (yes, Hotmail is still a thing!).

While you may think of those names being mainly used by individuals, many of them offer services packaged for companies to use — en masse with their employees — with one popular service being Google Workspace.

These email service providers have their built-in spam filters with advanced algorithms that look for the latest spam patterns and apply machine learning (ML) to evaluate a sender’s trust and the message’s inbox-worthiness.

2. Third-party spam filters

Third-party spam filters are additional layers of spam protection that companies add to their email system (whether onsite or hosted in the cloud). They work hand-in-hand with the company’s email infrastructure to provide a higher degree of fine-tuning, with the ability, for example, to choose between aggressive anti-spam measures (letting fewer emails in) and looser measures (allowing more emails in).

Companies that employ third-party spam filters also benefit from more robust reporting and analytics so they can better understand email and spam activity within the company. This can be useful for proactively thwarting targeted phishing attacks against a company.

3. Desktop spam filters

Desktop spam filters are pieces of software that you purchase and install on your computer. They monitor messages once they pass your email service provider’s spam filters and add an extra layer of protection. The level of security can be configured to suit your spam tolerance and tastes precise. As the filters built into email service providers become more and more sophisticated, desktop spam filters have become less popular.